X-ray scatterometry techniques are used for measuring geometrical structures of semiconductor devices. For example, U.S. Patent Application Publication 2015/0369759, whose disclosure is incorporated herein by reference, describes an apparatus including a source-mount that rotates about an axis, and an X-ray source on the source-mount that directs first and second incident beams of X-rays to impinge on a sample at first and second angles along beam axes that are orthogonal to the axis. An X-ray detector on a detector-mount receives first and second diffracted beams of X-rays transmitted through the sample in response to first and second incident beams, and outputs first and second signals, respectively, in response to the received first and second diffracted beams. A processor analyzes the first and the second signals so as to determine a profile of a surface of the sample measured.